The present invention relates to an exhaust gas recirculation (EGR) mechanism for engine and a method for controlling an exhaust gas recirculation valve. More particularly, the present invention pertains to a technique for optimally activating an EGR valve located in an EGR passage.
A typical engine has an EGR mechanism for reducing nitrogen oxide in exhaust gas by recirculating some of exhaust gas from an exhaust pipe to an intake pipe. The EGR mechanism includes an EGR passage connecting the exhaust pipe with the intake pipe and an EGR valve located in the EGR passage. The opening amount of the EGR valve must be finely controlled for optimizing the amount of the recirculation of exhaust gas. A typical EGR valve is a poppet valve, which includes a valve shaft and a valve body. The valve shaft is moved by an actuator in the axial direction. The valve body is secured to the distal end of the valve shaft and is located in the EGR passage. The valve shaft is supported by a seal member in the EGR passage.
As exhaust gas flows in the EGR passage, a deposit develops on a part of the valve shaft exposed in the EGR passage. The developed deposit is removed as the valve shaft slides on the seal member. However, the valve body is sometimes moved within a small range for a relatively long period without being fully opened. In this case, only a limited portion of the valve shaft slides on the seal member. Therefore, the deposit remains on part of the valve shaft that is located in the EGR passage and does not slide on the seal member. The remaining deposit is firmly adhered to the valve shaft. The adhered deposit cannot be easily removed even if the corresponding part of the valve shaft slides on the seal member when the valve shaft moves to fully open the valve body. The adhered deposit hinders smooth movement of the valve body and prevents the EGR valve from being accurately controlled.
To prevent valve shaft deposits, the EGR valve may be forced fully open when the engine is running. However, this would deteriorate the combustion of the engine, which would lower the power of the engine and the fuel economy and deteriorate the emissions. Thus, it is problematic to fully open the EGR valve for the purpose of removing deposits on the valve shaft while the engine is running.
Japanese Unexamined Patent Publication No 8-303307 discloses a device that moves an EGR valve between a fully open position and a fully closed position when an engine is stopped. Moving the EGR valve between the fully open position and the fully closed position when the engine is stopped completely removes deposits on the valve shaft. In other words, the device prevents deposits from adhering to the valve shaft.
However, if the engine is started again immediately after it is stopped, the engine is started with the EGR valve widely opened. This prevents the combustion chambers from getting sufficient oxygen, which makes the starting of the engine unsatisfactory.
Japanese Unexamined Patent Publication No. 7-293355 discloses a device that fully opens an EGR valve when an ignition switch is turned on for starting an engine. However, like the device of Publication No 8-303307, the device of Publication No. 7-293355 starts the engine with the EGR valve widely open and therefore hinders the starting of the engine.